1. Field of the Invention
The present invention comprises an apparatus for directly converting digital word input signals to RF voltage, the value depending on the state of the input. No intermediate analog circuitry is used. The RF DAC may be used to implement programmable attenuators and programmable phase shifters.
2. Problem to be Solved
Implementation of a high-speed (e.g. clock rates of over 100 MHz to over 1 GHz) programmable RF attenuator or phase shifter is beyond the state of the art with currently available components such as DACs, varactor diodes, PIN diodes and transistors, e.g. FETs, in either the linear or switching mode. Schottky diodes possess the properties of high-speed switching and attainable drive requirements; thus it is perceived to apply them to the problem. This results in an apparatus which produces the processed RF directly without intermediate analog steps.
3. Description of the Prior Art
Conventional DACs operate on direct voltage or current (DC) in converting digital word signals to baseband analog signals. For pulsed or continuous-wave (CW) applications involving radio-frequency (RF) signals, such as in a phase or amplitude modulator, a control signal is first generated with a conventional DAC and then passed on to an RF signal processor (for example, a varactor diode phase shifter, or a PIN diode or FET attenuator). DC DACs are expensive and require high power. The state of the art of conventional DAC is epitomized by the TriQuint TQ6112M which settles to 1% in 1 nanosecond.
Examples of analog RF signal processors are varactor diodes, PIN diodes and FETs. Varactor diodes, whose capacitance depends on the applied reverse voltage, are used as phase shifters. PIN diodes, whose resistance depends on the value of forward current, may be used as attenuators. Field-effect transistors (FETs), whose resistance depends on the value of the gate voltage, may also be used as attenuators. The varactor and PIN diodes have slow (greater than 1 nanosecond) response times. The response speed is limited by the device itself as well as the speed of the driver. In addition, they require high power to drive high capacitance and low impedance. The drivers themselves have response times of several nanoseconds.
FETs are easier to drive, but still have slow response times in analog circuits. Examples of FET attenuators are the HARRIS HMR-11000 (3-ns typical response time) and the HP HMMC-1001 (1-ns response time).